Conventionally, as the methods of developing the electrostatic latent image formed on the image bearer in an image forming apparatus using the electro-photographic method, known are the one-component developing system which uses only a toner as the developing agent and the two-component developing system which uses toner and a carrier.
Generally, in the one-component developing system, the toner is charged by passing through a regulating section that has a toner supporting member and a regulating plate that is presses by the toner supporting member, and also, it is possible to obtain the desired thin layer of the toner. Because of this, it is advantageous in terms of simplification of the apparatus, size reduction, and achieving low cost. However, it is easy for toner deterioration to progress due to the strong stress of the regulating section, and also it is easy for the charge receiving property of the toner to become lower. In addition, because the surfaces of the regulating member and the toner supporting member, which are members applying charge to the toner, get contaminated by the toner or the external additive agents, even the property of applying charge to the toner gets reduced. Therefore, the amount of charge on the toner decreases, causing problems such as fogging, and hence the life of the development apparatus is short.
On the other hand, in the two-component development system, since the toner is charged by friction charging due to mixing with the carrier, the stress is small, and this method is very effective against toner deterioration. In addition, even the carrier which is the material applying electric charge to the toner, because its surface area is large, is relatively strong against contamination due to toner or external additive agents, and this method is advantageous in terms of life. However, even when a two-component developer is used, the surface of the carrier does get contaminated by the toner and the external additive agents, the amount of charging of the toner gets reduced over a long time of use, and problems such as fogging or toner splashing occur. Because of this, its life can not be said to be sufficient, and still longer life is desired.
In view of this, several proposals have been made of technologies that suppress the deterioration of the carrier and make the life longer of two-component developers (see, for example, Japanese Laid-Open Patent Application Publication No. S59-100471 and No. 2003-215855).
In Patent Japanese Laid-Open Patent Application Publication No. S59-100471 disclosed is a development apparatus that suppresses the increase in the ratio of deteriorated carriers by replenishing the carrier gradually in the developer together with the toner or independently, and the replacement of carrier is carried out in accordance with that by discharging the deteriorated developer the charging property of which has gone down.
Further, in Japanese Laid-Open Patent Application Publication No. 2003-215855, disclosed are a two-component developer having carrier and toner which is externally added with particles having the property of being charged to a polarity opposite to the charging polarity of the carrier and a development method using this developer.
However, in the development apparatus disclosed in Japanese Laid-Open Patent Application Publication No. 2003-215855, since the carriers are being replaced, it is possible to suppress the reduction in the amount of charging of the toner due to carrier deterioration to a fixed level, and this is advantageous in obtaining a long life. However, there are problems in the aspects of cost and environment because a mechanism for retrieving the discharged carrier is necessary, and because the carrier becomes a consumable item. In addition, it is necessary to repeat printing of a prescribed amount until the ratio of old to new carriers becomes stable, and it is not necessarily possible to maintain the initial characteristics.
Further, in Japanese Laid-Open Patent Application Publication No. 2003-215855, it has been indicated that particles with opposite polarity charging property are added with the intention of acting as polishing material and spacer particles, and that there is the effect of suppressing deterioration due to the effect of removing the spent matters on the surface of the carrier. In addition, it is said that there is the effect of improving the cleaning in the image bearer cleaning section and of polishing the image bearer. However, in the disclosed development method, the amount of consumption of the toner and the opposite polarity charging particles differs depending on the image area ratio, particularly when the image area ratio is small, the consumption of the opposite polarity charging particles adhered to the large area non-image area becomes excessive, and there is the problem that the effect of suppressing the carrier deterioration in the development apparatus becomes lower.
In view of this, in order to retain the features of both the development methods, a combined development method (hereinafter referred to as a hybrid development method) appeared that uses a two-component developer in which a non-magnetic toner is charged using a magnetic carrier, and in order to develop the electrostatic latent image formed on the photoreceptor which is the image bearer, the charged toner is separated selectively from the two-component developer and retained on the development roller. Since this hybrid development method can develop by forming a dense toner layer on the development roller and developing in a state of close proximity with the photoreceptor, it is possible to carry out particularly fast image forming, and also, the stress applied to the developer and the development roller is small, and has attracted a lot of attention as a method that can offer long life.
However, while the hybrid development method has the above advantages, on the other hand it came to be known that it also has the following problems.
That is, a toner selection phenomenon occurs in which a toner with a high developing capacity (a toner that can adhere easily to the electrostatic latent image surface due to the developing electric field strength) is easily developed selectively but the toner having a large amount of charge is not consumed but remains on the development roller, and as a consequence, when carrying out successive printing, there is the problem that the image density decreases successively. In addition, there is the problem that the pattern of the previous image appears as a residual image (ghost) at the time of forming the next image.
To counter this problem, a method has been proposed, for example in Japanese Laid-Open Patent Application Publication No. 2002-108104, in which an equipotential state is generated to eliminate the potential difference between the development roller and the feed roller either during the non-image forming period or before starting the image forming, thus decreasing the adhesion force of the toner on the development roller and recovering the residual toner.
Further, for example, in Japanese Laid-Open Patent Application Publication No. 2005-189708, a counteracting method has been proposed of definitely separating the magnetic brush formed on the feed roller using a stirring member by stipulating the positional relationship between the development roller and the feed roller or the amount of two-component developer on the feed roller.
Further, for example, in Japanese Laid-Open Patent Application Publication No. 2000-298396, a method has been proposed of peeling off the residual toner layer after development by making a toner peeling off member come into pressure contact with the development roller.
However, in Japanese Laid-Open Patent Application Publication No. 2002-108104, since a non-image forming period is required, when carrying out image formation successively in high speed, it will not be possible to carry out sufficiently the recovery of residual toner in the period between the previous image and the next image (between images). Further, there is the problem that the printing speed gets reduced if the interval between images is made long. In addition, in Japanese Laid-Open Patent Application Publication No. 2005-189708, although the completeness of the separation of the developer on the feed roller by the stirring member gets improved, it is not possible to sufficiently recover the residual toner on the development roller, and the residual image, which is the pattern of the previous image, remains on the next image. Also, in Japanese Laid-Open Patent Application Publication No. 2000-298396, the drive torque of the development roller becomes high due to the pressure contact of the toner peeling off member, thereby making the motor larger and increasing the cost. In addition, there will be friction of the pressure contacting member and scratches on the development roller thereby causing reduction in the life of the product and noise in the images.